1. Field of the Invention
The present invention relates to the field of flexible or supple materials made of expanded or foamed polymer which can be used in particular for the preparation of leaktight, insulation and/or damping components.
2. Description of the Background
In this field, synthetic materials in the form of foam or having a cellular structure, manufactured as a broad strip or as a strand, which is cut to the desired dimensions and which is applied to the corresponding surface via an adhesive layer, are known. A material widely used is PVC foam manufactured from a plastisol to which a foaming agent has been added. However, this is difficult to implement when the surface has a complex geometry. It can also take too long to carry out in the context of large scale mass production of parts.
For these applications, in particular in the automobile industry or in industries for the manufacture of various electrical devices, a technique has been developed for the production of a gasket foamed in situ or in place (formed in place foam gasket or foamed in place gasketxe2x80x94FIP) by deposition in place of a material with an appropriate viscosity which changes into a foam by crosslinking in the open air. The material can be applied into a groove, in a form or on a smooth surface in the case of thixotropic or three-dimensional systems.
A first alternative form of this technique uses, as the material to be deposited, a system with two components (two-component system) each of which are stored separately from one another and mixed in appropriate amounts just before application by devices for metering and mixing under reduced pressure. Two-component systems are known for forming silicone or polyurethane foams. This technique is disclosed in particular in EP-A-0,416,229.
A second alternative form of this technique eliminates the disadvantages related to metering and mixing at the time of use by using a so-called single component system: the material to be deposited is prepared in advance and is provided in a stable form which can be stored under an inert atmosphere until use.
A typical composition suited to this use is disclosed in EP-A-0,326,704. It comprises a first prepolymer component, which can self-crosslink with water, as an intimate mixture with a second noncrosslinked elastomer component, so as to constitute, after extrusion and crosslinking, a material of the interpenetrating polymer network type. Depending on the viscosity of the mixture and the treatment conditions, the extruded substance can form the foam spontaneously or the foaming can be obtained by virtue of a chemical or physical agent. An example of equipment suitable for extruding this substance in the presence of a foaming gas is disclosed in U.S. Pat. No. 4,405,063.
Although this technique relatively easily produces a foamed gasket having qualities which are sufficient for applications in sealing, insulation or others as mentioned above, it can be further improved in order to achieve improved performance.
An object of the present invention is to provide a composition of substances having an improved ability to foam which makes it possible in particular to obtain products of relatively low relative density with mechanical properties at least as good as those of known products.
This object, as well as others which will become apparent subsequently, are achieved by using, as extrudable composition, a single-component product comprising a single macromolecular constituent containing a polyurethane prepolymer comprising isocyanate or trialkoxysilyl end groups which is self-crosslinkable with moisture.
Against all expectation, the Inventors found that it is possible to form a satisfactory foamed product without resorting to an interpenetrating network structure according to EP-A-0,326,704, in which structure the elastomer component serves to impart to the material the elasticity necessary for its mechanical strength, while the crosslinkable component contributes the thermosetting nature to the material. On the one hand, a composition comprising only a thermosetting polyurethane prepolymer lends itself perfectly to the technique of extrusion in the presence of pressurized gas: it does not present any problem of a rheological nature at the outlet of the extrusion nozzle, and the composition forms, fairly rapidly and without sagging, a foam which virtually instantaneously acquires dimensional characteristics very similar to the definitive characteristics. The flexible cellular material may therefore not include a noncrosslinked elastomer component, or the only organic polymer in the flexible cellular material may be one or more polyurethanes. Likewise, the polyurethane prepolymer may be the sole organic polymer in the extrudable composition, or the extrudable composition may not include a second elastomeric polymer.
On the other hand, under optimum manufacturing conditions, a composition comprising only a polyurethane prepolymer forms a foam with a lower relative density than a composition additionally comprising at least one other macromolecular constituent. The same volume of foamed material is thus obtained with a reduced amount of substance. A substantial saving in substance is achieved while retaining mechanical characteristics, in particular flexibility characteristics, which are sufficient for applications such as leaktight or insulation seals.
The present invention produces cellular materials for which the density (after crosslinking) is less than 300 kg/m3, in particular on the order of 260 kg/m3 or less, in particular on the order of 250 kg/m3 or less, for example less than or equal to 200 kg/m3.
The cellular material obtained is flexible with, advantageously, a substantially elastic behaviour. The material can generally have a compression set at room temperature of less than 25%, advantageously on the order of 15% or less, in particular of less than or equal to approximately 10%, for example on the order of 5% or less. A low compression set indicates a good ability of the material to withstand compression. The values indicated above are compatible with a lasting leaktight capability in the usual applications.
Furthermore, the material as a crosslinked foam generally exhibits a smooth skin and a relatively fine to very fine cellular structure, which indicate the ability of the single-component product to prevent the bubbles of gas from bursting at the free surface of the extruded substance while preventing the coalescence of the bubbles of gas within the material. These characteristics make the material entirely suited to applications in leaktightness and/or insulation.
The cellular structure is advantageously such that the cells have a dimension of less than 0.3 mm, preferably of less than 0.2 mm. Structures comprising cells with a very small dimension, for example with a size of less than 0.1 mm, are particularly advantageous. xe2x80x9cFinexe2x80x9d describes a structure in which the cells have dimensions of between approximately 0.1 and 0.3 mm, and xe2x80x9cvery finexe2x80x9d describes a structure in which the cells have dimensions of between approximately 0.03 and 0.2 mm.
In addition, the Inventors have demonstrated the fact that the foaming increases as the polymer system of the single-component product exhibits, at the supramolecular level, a reduced number of phases. Preferably, the polyurethane prepolymer forming the macromolecular constituent of the single-component product is essentially single-phase.
xe2x80x9cEssentially single-phasexe2x80x9d means a polymer system in which the macromolecular chains are essentially miscible. This is in particular the case when the polyurethane prepolymer is a homopolymer, the macromolecular chains being arranged in a single, perfectly homogeneous phase. This can also be the case when the prepolymer is a random copolymer. This can still be the case when the prepolymer is a block or grafted copolymer in which the various blocks (derived from at least two distinct monomers) are miscible with one another, optionally in a specific range of relative proportions. The blocks are preferably completely miscible with one another, so that a single homogeneous phase is observed by microscopy, but they can also be arranged so that (at least) one acts as a xe2x80x9cmatrixxe2x80x9d, the other being finely dispersed in the first. The latter structure, which exhibits a single continuous polymer phase (the xe2x80x9cmatrixxe2x80x9d), is covered by the meaning of xe2x80x9cessentially single-phasexe2x80x9d in the present invention.
In contrast, the expression xe2x80x9cessentially single-phasexe2x80x9d as used here excludes systems in which the macromolecular chains separate into at least two cocontinuous phases, that is to say divided into as many macroscopic domains.
Depending on the types of block desired, a person skilled in the art is in a position to determine, using simple microscopic observations and without undue experimentation, the fractions by mass to use in order to be in the corresponding chain miscibility range.
The polyurethane prepolymer is a noncrosslinked oligomer, preferably with a molecular weight of less than 20,000 g/mol, obtained by reaction between (i) at least one component of polyol or polyamine type and (ii) at least one polyisocyanate component, optionally followed by a reaction for protecting the end functional groups by a trialkoxysilane.
The reactant (i) is advantageously chosen from polyols and polyamines with a functionality at least equal to 2 having a backbone of polyester, polycaprolactone, polyether, polyolefin, in particular hydroxlyated EVA copolymer, saturated or unsaturated polybutadiene, polyisoprene or polydimethylsiloxane type.
The backbone is preferably of type:
aliphatic and/or aromatic polyester, preferably essentially aliphatic, derived in particular from aliphatic glycols, optionally diethylene glycol, and from aliphatic and/or aromatic acids; or
polyether, in particular poly(ethylene oxide) and/or poly(propylene oxide) or polytetrahydrofuran.
The polyol or polyamine component is advantageously an oligomer with a molecular mass of less than or equal to approximately 10,000 g/mol, preferably on the order of 500 to 4000 g/mol, in particular from 1500 to 3500 g/mol. Its functionality is preferably on the order of 2 or more, in particular on the order of 2 to 2.5.
Moreover, the reactant (ii) can be chosen from simple molecules, in particular aromatic molecules, carrying at least two isocyanate functional groups and oligomers (with a molecular mass which can be chosen within the ranges indicated above) comprising isocyanate end groups with a functionality at least equal to 2. The reactant advantageously comprises at least one polyisocyanate component with a functionality at least equal to 2 of low molecular weight chosen from para-phenylene diisocyanate, trans-1,4-cyclohexane diisocyanate, 3-isocyanatomethyl-3,3,5-trimethylcyclohexyl isocyanate, 1,5-naphthalene diisocyanate, 4,4xe2x80x2-methylenebis(phenyl isocyanate) (pure MDI), crude MDI, 2,4-toluene diisocyanate (2,4-TDI), 2,6-toluene diisocyanate (2,6-TDI) and their mixtures (for example, 80/20 TDI comprising 80% of 2,4-isomer or 65/35 TDI), and crude TDI (unpurified 80/20 TDI). Among these components, crude or pure MDI is very particularly preferred.
For the polyisocyanate component, the functionality is preferably on the order of 2 or more, in particular on the order of 2 to 2.8.
For the purpose of obtaining a material in which the polymer matrix is a system with few phases, it is advantageous to choose a polyisocyanate component comprising a non-oligomer backbone, that is to say a low molecular weight aliphatic or aromatic component in which the isocyanate groups are not bonded to a polymer backbone.
For the purpose of obtaining a single-phase system, it is very particularly advantageous to react a single type of polyol or polyamine component with a low molecular weight polyisocyanate component. xe2x80x9cA single typexe2x80x9d is understood to mean that the chains of the oligomer backbone of the component belong to a single polymer family, while optionally being able to combine several members of this family. For example, it is possible to use a polyetherdiol in which the polyether chains are composed exclusively of poly(propylene oxide) but also optionally a mixture of (poly(propylene oxide))diol and of (poly(ethylene oxide))diol.
When use is made of an oligomer comprising isocyanate end groups, its chemical structure and/or its fraction by mass in the mixture are preferably chosen so that the macromolecular chains of the two components (i) and (ii) are miscible with one another.
The relative amounts of polyol/polyamine component (i) and of polyisocyanate component (ii) are chosen so as to allow the production of a stable polyurethane prepolymer exhibiting isocyanate end groups. The polyisocyanate excess is preferably chosen so that the molar ratio of the isocyanate groups NCO to the alcohol groups OH and/or amine groups NH2 (NCO/OH+NH2) is on the order of 2 to 3.5.
The reaction time and temperatures vary according to the components used, their determination for each specific case being well within the skill of a person skilled in the art, without undue experimentation.
A prepolymer comprising trialkoxysilyl end groups can be obtained from the reaction product of the components (i) and (ii) by subjecting this product to a reaction with a trialkoxysilylating agent. A trialkoxysilane capable of reacting with an NCO group can be a trialkoxyaminosilane, for example an aminopropyltrialkoxysilane, such as aminopropyltrimethoxysilane, or a trialkoxymercaptosilane. The prepolymers terminated by isocyanates are nevertheless preferred in so far as their self-crosslinking in the presence of water is much faster.
The cellular material according to the invention can be composed exclusively of polyurethane but its polymer matrix can also comprise a filler. Filler is generally understood to mean here a product which is neither soluble nor miscible in the polymer matrix, which is dispersible in the latter and which allows one or more properties or characteristics (mechanical or chemical properties, colour, production cost) of the final mixture to be improved. To this end, the extrudable single-component product can additionally comprise an organic or inorganic, particulate or pulverulent filler, for example calcium carbonate and/or carbon black. The single-component product can also comprise conventional additives, such as plasticizer, colorant, stabilizer, cell regulator, catalyst, and the like.
Another subject-matter of the invention is a stable composition which can be extruded in the presence of pressurized gas in order to form a flexible cellular material having an expanded or foamed polymer matrix, this composition comprising a macromolecular constituent, characterized in that the macromolecular constituent is a polyurethane prepolymer comprising isocyanate or trialkoxysilyl end groups which is self-crosslinkable with moisture. This type of composition generally has a fairly low viscosity which can in particular be less than 500 Pals at less than 60xc2x0 C., which greatly facilitates shaping by extrusion. It is stable on storage under a dry atmosphere.
Another subject-matter of the invention is a process for the manufacture of a cellular material as described above. This process comprises:
preparing a single-component product of the above composition,
optionally storing the single-component product away from moisture, in particular under a dry atmosphere or under vacuum,
mixing the product with a pressurized gas in order to form an extrudable substance,
extruding an amount of extrudable substance,
crosslinking the extruded substance in a moist atmosphere.
The cellular material according to the invention is preferably manufactured in the form of a strip, panel, strand or pipe for a leaktight seal. It can be produced by direct extrusion over the surface provided for its application or else by extrusion in a mould carrying the negative impression of the surface in question and then transfer onto this surface.
Extrusion is understood to mean here, within the broad sense, a technique in which a substance in the fluid or viscous state is conveyed to an applicational orifice which we shall call a nozzle. This term does not restrict the invention to a technique for configuring the substance, the latter being free to adopt, at the outlet of the orifice, dimensions substantially different from those of the cross-section of the nozzle.
The gas can preferably be nitrogen, but also any other gas known for this purpose: air, carbon dioxide, n-pentane, and the like.
The moist crosslinking treatment can be carried out under conditions known to a person skilled in the art, for example in a temperature range from room temperature to approximately 80xc2x0 C. in an atmosphere having a relative humidity on the order of 40 to 100%.
The crosslinking can be accompanied by a swelling of the material due to the release of CO2 by the crosslinking reaction of the free isocyanate groups with the water. Generally, a high relative humidity promotes a high degree of swelling.